Diatomics-in-molecules description of the Rg–Hal₂ rare gas–halogen van der Waals complexes with applications to He–Cl₂

The diatomics-in-molecules (DIM) technique is applied for a description of the low-lying states of the Rg–Hal₂ van der Waals complexes correlating with the lowest states of constituent atoms Rg(¹S)+Hal(²P_j)+Hal(²P_j). The important feature of this approach is the construction of polyatomic basis functions as products of the Hal₂ diatomic eigenstates classified within the Hund “c” scheme and the atomic rare gas wave function. Necessary transformations to the other basis set representations are described, and finally all the matrix elements are expressed in terms of nonrelativistic adiabatic energies of Hal₂ and Rg Hal fragments and spin-orbit splitting constant of the halogen atom. Our main concern is to test the DIM-based approximations of different levels taking the He–Cl₂ system as an example. Namely, we have compared the results obtained within a hierarchy of approaches: (1) the simplest pairwise potential scheme as a far extreme of the DIM model, (2) the same as (1) but with the different components (Σ and Π) for He–Cl interaction, (3) the accurate DIM technique without spin-orbit terms, and (4) the highest level which takes into account all these contributions. The results have been compared to the other DIM like models as well. The shapes of two-dimensional potential surfaces for the ground (X) and excited (B) states of HeCl₂, binding energies D_e with respect to He+Cl₂, stretching and bending vibrational frequencies of the complex, binding energies D₀, and spectral shifts for the B←X transition are discussed.